The process measurement industry employs process transmitters to remotely monitor process variables associated with substances such as solids, slurries, liquids, vapors, and gasses in chemical, pulp, petroleum, pharmaceutical, food and other processing plants. Process variables include pressure, temperature, flow, level, turbidity, density, concentration, chemical composition and other properties. A process transmitter is a transducer that responds to a process variable and converts the variable to a standardized electrical signal.
A process transmitter communicates the standardized electrical signal over a process loop to a control room, such that the process can be monitored and controlled. One type of process loop is a two-wire, 4-20 mA process control loop. Two-wire process transmitters operate on such low energy levels that they receive all electrical power from the 4-20 mA loop.
Process transmitters have evolved from devices having only analog components, to integrated smart transmitters having analog and digital components. Each new generation of process transmitters is expected to provide higher performance and more functionality than the previous generation. As performance requirements of process transmitters increase, the basic power requirements of the transmitters remains unchanged. For example, for new generations of 4-20 mA process transmitters, a common requirement that the electronics draw less than 3 mA typically still applies.
To provide increased functionality, some process transmitters are being designed with low voltage component technologies that reduce current consumption by the electronics. The supply or rail voltages used by a component are the direct current (D.C.) voltages provided for powering the component. With the current saved by reducing the supply voltage, performance and functionality can be increased without exceeding 3 mA of total current consumption.
Generally, digital components require lower supply voltages than analog components. However, analog components are needed to provide high resolution information on the sensed variable. Analog components do not operate well at low voltages. Also, a reduction in the supply voltage limits the input voltage range for analog-to-digital converters which are frequently included in process transmitters. Limiting the input voltage range of an analog-to-digital converter limits the resolution of the analog-to-digital converter. The combined effect of the decrease in signal to noise ratio and the limited analog-to-digital converter input voltage range can result in a significant loss of overall performance. Further, each time new lower supply voltage generations of digital components are implemented in a process transmitter, the analog sensor electronics must typically be redesigned.
A step-up converter for powering analog components in a process transmitter permits powering both low voltage digital components and higher voltage analog components. A step-up converter is a switching regulator which receives an input voltage signal and outputs a signal having a higher voltage. The process transmitter includes a power regulator which provides the input voltage signal to the step-up converter. The step-up converter receives the input voltage signal and outputs the higher voltage signal. Analog components electrically coupled to the step-up converter receive power from the higher voltage signal, while digital components electrically coupled to the power regulator receive power from the input voltage signal.
The step-up converter included in the process transmitter allows low voltage digital components to be utilized, while also allowing analog components to operate at higher voltages. Operating the analog components at higher voltages improves the resolution and other performance characteristics of the transmitter. Also, inclusion of the step-up converter allows new lower voltage digital components to be used in the future, without redesigning the analog components.
In one aspect, a start-up circuit is included in the process transmitter that prevents the step-up converter from providing the higher voltage signal until the input voltage signal has surpassed a first threshold voltage. The start-up circuit prevents the step-up converter from drawing excessive amounts of current during initialization or power-up of the transmitter.
In another aspect, the start-up circuit controls a switch to selectively connect the step-up converter to analog components. The start-up circuit controls the switch such that the signal having the higher voltage is used to power the analog components only after the voltage of the input voltage signal has surpassed a second threshold voltage. The second threshold voltage is higher in magnitude than the first threshold voltage. The start-up circuit prevents the step-up converter from being connected to a load prior to the input voltage signal surpassing the second threshold in order to increase the power efficiency of the step-up converter.
Still other aspects include a start-up circuit or circuits that both prevent the start-up circuit from providing the higher voltage signal until after the input voltage signal has surpassed the first threshold, and control the switch to connect the higher voltage signal to the analog components only after the input voltage signal has surpassed the second threshold.